The present invention relates to the formation of high uniformity in needled structures particularly for use as reinforcing structures in forming rocket motor parts, heat protective pieces and high performance brake discs or pads as used on aircraft and land vehicles.
Brake discs of the type presently used in jet aircraft and Formular One racing must endure very high braking loads and the resulting shear forces that are experienced in the brake disc material. In the case of aircraft brakes, the thickness of the disc may be two inches thus accentuating the shear on the disc material.
To withstand such shear forces, the discs must be manufactured to great accuracy so that there is as much uniformity in the disc material as is possible. A nonuniform disc material would create local stress concentrations that will increase the possibility of pad failure.
One method of manufacturing such brake disc in the past has used the technology of needling together a plurality of layers of a fibrous material which is then carbonized and densified by a matrix material, for example by carrying out a chemical vapor deposition process, to obtain a composite brake disc. The uniformity of the resulting brake disc is a function of the uniformity of the needling process. Prior needling processes have however suffered from lack of uniformity resulting from the fact that in bonding together a thick stack of many layers, the needling effects differently the layers at the top than those at the bottom. As the needles used in needling penetrate the layer of material, they pick up and become clogged with the fibers of the fibrous material. As a result, the needles become increasingly less effective in producing the interlayer fiber crosslinking as they penetrate the stack of layers. The result is a non-homogeneously needled stack. In the critical applications of high performance brake discs, it is then impossible to optimize the needling throughout the layers. If it is optimized at the top of the stack, the needling variations throughout the stack will produce suboptimal results at the bottom, and vice-versa. It thus becomes difficult to realize the required disc strength throughout its thickness.
With regard to the prior art, a known process for manufacturing flat structures by needle superposed layers of fibrous fabric is described in U.S. Pat. Nos. 3,971,669 and 3,994,762. According to this known process, flat unidirectional layers are superposed in criss-crossed fashion and then needled. Although no limitation is given as to the number of superposed layers, this document gives no indication as to the means that should be used for producing a thick structure with homogeneous characteristics therethrough.
French Pat. No. 2,414,574 discloses a process for manufacturing fibrous reinforcements for brake discs, by forming rings of felt by needling, stacking the rings one over the other until the required thickness is reached, and maintaining the resulting stack for eventual densification. It is indicated that the stack of rings can be needled, but no description is given as to what method is used to this effect.
British Pat. No. 1,549,687 also discloses a process for manufacturing fibrous reinforcements for brake discs. Annuli of a fibrous material are plied together to form a stack which is passed through a needle-punch loom. Further series of plies may be added to the initially needled stack and the resulting stack is passed through the needle-punch loom. The needle penetration can vary from one needling pass to the other, but the result does not appear to be an homogeneously needled structure. In addition, the thickness of the stack is limited by the height of the loom. Compression of the stack between needling passes, as suggested, will not help in obtaining structures with uniform needling density and with unlimited thickness.
Regarding now the manufacturing of cylindrical structures by needling layers of fibrous fabric, British Pat. No. 2,099,365 describes a process which consists in winding a sheet of fibrous fabric on a cylindrical mandrel and carrying out a needling operation on the sheet while on the mandrel. This document, however, gives no indication as to what means to use in order to effectively produce a thick structure with a constant needling density right through the thickness of the structure. The same applies to French Pat. No. 2,378,888 and to U.S. Pat. No. 3,772,125, and to the prior art documents relating to the production of tubular structures
thin walls (such as for example FR No. 1,570,992, GB No. 2,048,424 and U.S. Pat. No. 3,909,893).
Now it has been found that the technique of needling through small thicknesses is not readily adaptable to large thicknesses. One reason for this is that, once they have penetrated to a certain thickness in the superposed layers, the needles loose their effectiveness because their barbs become clogged up with pieces of fibers torn from the layers of materials gone through by the needles; the needles cannot thus fulfill their function correctly, and as a result, the same needling characteristics cannot be obtained throughout the whole stack.
Yet structures destined to be subjected to strong thermo-mechanical stresses as in brake discs, it is important to keep the properties constant throughout the structure, in order for example, to avoid delamination.
It is the object of the present invention to propose a process permitting the production of thick three-dimensional structures by needling of superposed layers, with a constant density of needling throughout the thickness of the structure, to allow the production of homogeneous fibrous structures with a constant density of fibers throughout the structures.